During the drilling of a well, mud is circulated downhole to carry away drill cuttings. Should gas be encountered, it becomes incorporated with the mud and is conveyed to the surface. In an active mud system, the mud is circulated in a loop; pumped from the mud tank, downhole to the bit, up to the surface, and back to the mud tank.
As the mud flows to the mud tank, an agitator, placed in the mud stream, causes contained gas to be liberated from the mud.
The liberated gas is directed past a gas sensor. One type of gas sensor is gas chromatography which produces a record of the constituents of the gas. Unfortunately, chromatography apparatus and methods of using same obtains only discrete analyses of gas in batches. A gas sample is occasionally selected and tested by the chromatograph. By the time the chromatograph is ready for the next sample, the drilling may have travelled a further ten feet or so and passed through and beyond a formation of interest. When the subsequent sample is obtained, the formation may then be uninteresting.
For producing a continuous gas trace, it is generally known to use a catalytic, rare earth or hot wire gas sensor. The sensor detects the presence of combustible gases. These devices are also called explosimeters and indicate the relative fraction of volatile hydrocarbons in a gas steam. Often these apparatus are used to determine if a gas mixture may be explosive.
The conventional gas sensor is a rare earth (hot-wire) sensor. An electrical current is passed though the sensor. The sensor heats up and dissipates energy dependent upon its ability to exchange energy with the surrounding environment. In these applications it is the gas flow and gas composition which affects the heat dissipation. Heat or power dissipation results in a change in the resistance of the sensor.
The sensor is epoxy coated for limiting the sensor from thermal effects and for excluding chemical interaction with the sensor's rare-earth portion.
The sensor output is recorded as a trace on a strip chart recorder or digitally on a computer and output for viewing on a screen. The presence of combustible gas shows up as an analog voltage output.
The difficulty with the prior art predominately lies in the interpretation of the continuous gas sensor output. This output responds to a high concentration of a predominantly methane gas with an output similar to a lesser amount of a heavier hydrocarbon.
There is therefore a demonstrated need for a real-time system which is capable of distinguishing heavier hydrocarbons (indicative of oil) from lighter hydrocarbons (representing coal gas or methane) while drilling, thereby affording the drilling operator an onsite ability to assess the value of the well.